Power over LAN™, or Power over Ethernet, is a new technology that enables DC power to be supplied to Ethernet data terminals over ordinary local area network (LAN) cabling such as Category 5 cabling. This technology enables the terminals, termed powered devices (PDs), to receive their operating power over the same Ethernet LAN connection that they use for data communication. It thus eliminates the need to connect each terminal to an AC power socket, and to provide each terminal with its own AC/DC power converter. The technology also enables PDs to be recognized as such by a “signature” generated by the terminal. Further aspects of this technology, including a capacitor signature detection method, are described in PowerDsine Application Notes 115 and 111, entitled “Power over LAN™: Building Power Ready Devices” and “Analysis of the PD/Power over LAN™ Hub requirements” published by PowerDsine Ltd., Hod Hasharon, Israel, which are incorporated herein by reference. The LAN MAN Standards Committee of the IEEE Computer Society is developing specifications for Power over LAN systems, as described in IEEE Drafts P802.3af/D3.0 or later, entitled “Data Terminal Equipment (DTE) Power via Media Dependent Interface (MDI)” (IEEE Standards Department, Piscataway, N.J., 2001), which is also incorporated herein by reference. The specifications are referred to herein as standard 802.3af.
A Power over LAN system comprises an Ethernet switch and a power hub, which serves as the DC power source, along with a number of PD terminals, which communicate via the switch and draw power from the hub. The system is typically connected in a star topology, with each terminal linked by a cable to the switch and hub. The power hub in one chassis may be integrated with the switch in a second chassis, in a console containing both chassis, in what is known as an “end-span” configuration. Alternatively, the power hub chassis may be located between the switch chassis and the terminals, in a “mid-span” configuration. DC power is carried to the loads (i.e., the terminals) over twisted pairs provided by Category 5 cabling. The end-span configuration uses twisted-data-pairs that are also used for Ethernet communication; the mid-span configuration uses spare twisted-spare-pairs that are not used for Ethernet communication.